Nozzle assembly for preventing back-flow

ABSTRACT

A nozzle assembly which comprises a nozzle aperture for discharge of a fluid as a spray of droplets, and a conduit in fluid flow communication with the nozzle aperture. The flow of fluid through the conduit is restricted by the minimum effective cross-sectional area of the conduit transverse to the line of flow of fluid through the conduit so that back flow of fluid from the nozzle aperture through the conduit at ambient and operational pressure differentials is substantially prevented. The invention further provides for a method of discharging a fluid as a spray of droplets, and a spray generating device which includes the nozzle assembly of the present invention.

This invention relates to a valve, notably to a non-return valve whichcan also act as a filter for use in devices for forming sprays ofdroplets.

BACKGROUND TO THE INVENTION

Many forms of device have been proposed for dispensing fluids, forexample medicaments, as sprays of fine droplets or aerosols. In someforms of device, it has been proposed that the aqueous solution of themedicament or other active ingredient be discharged through a fineorifice nozzle to form the spray using mechanical pressurizing means,for example using a compressed spring to drive a piston in a cylindercontaining the fluid; in others, a pressurized gas is used as thepropellant. For convenience the term pressurizing means will be usedherein to denote all means by which the pressure required to dispensethe fluid is generated and includes mechanical and pressurized gasoperated means.

Where very small nozzle apertures, for example those having a diameterof 10 micrometers or less, are used to form fine droplets sizes, it isimportant to ensure that such small orifice apertures do not becomeblocked. It has therefore been proposed to provide a filter in the fluiddischarge line upstream of the nozzle aperture. Small dimension filtersare available, and these typically comprise a mesh or gauze which has amesh aperture size as low as 3 micrometers or less. However, suchfilters are flimsy and therefore require some support means to preventrupturing under the large pressures generated by the pressurizing means.Furthermore, such filters and their support means are additional andoften expensive components.

There is, therefore, a continuing requirement for an effective andreliable filter capable of filtering fluid stream down to a very smallparticle sizes. In spray generating devices, there is usually also arequirement for a non-return valve positioned between the pressurizingmeans and the atomizing nozzle orifice so as to reduce the risk ofresidual fluid in the nozzle assembly draining back into thepressurization chamber and contaminating fluid held in a reservoir inthe device.

We have devised a form of nozzle assembly incorporating a non-returnvalve assembly which provides a simple and effective means for reducingthe risk of drain back of fluid from the nozzle assembly and may also beused to provide the functions of a filter and/or a filter gauze supportin the nozzle assembly.

SUMMARY OF THE INVENTION

Accordingly, from one aspect, the present invention provides a nozzleassembly comprising a conduit in fluid flow communication with a nozzleaperture through which fluid is adapted to be discharged as a spray ofdroplets, characterised in that the effective minimum cross-sectionalarea of the conduit transverse to the line of flow of fluid at thatpoint is selected so that flow of fluid through said conduit isrestricted by the minimum effective cross-sectional area wherebyback-flow of fluid from said nozzle aperture through said conduit at anambient pressure differential and at an operational pressuredifferential is substantially prevented.

The invention also provides a spray generating device incorporating anozzle assembly of the invention.

The term effective is used herein with respect to the cross-sectionalarea of the conduit to denote that cross-section of the conduit which isnot occupied by an infill or other member, and through which fluid mayflow. Thus, the conduit may be a fine bore tube, in which case theeffective cross-sectional area is the cross-section of the fine bore.However, the conduit may also be in the form of a wide bore chamber intowhich is fitted a solid or hollow plug which reduces the freecross-sectional area of the chamber through which fluid can flow.

For convenience, the term upstream will be used herein to denote thedirection opposed to a flow of fluid from the conduit to the nozzleaperture; the term discharge flow to denote a flow of fluid from theconduit to the nozzle aperture; and the term back flow to denote a flowof fluid from the nozzle aperture back to the conduit.

The clearance or passageway(s) within or between components of thenozzle assembly forming the conduit through which the flow of fluid isrestricted acts to minimize the back flow of fluid in the nozzleassemblies of the invention during the rest state of a spray generatingdevice incorporating the nozzle assembly or when suction is applied tothe nozzle assembly as the pump or other means for discharging the sprayis re-cocked after use. During the rest state there will usually be nopressure differential across the nozzle assembly and it will be thesurface tension effects at the nozzle aperture and the flow resistancecaused by the walls of the passageway(s) which restrict back flow offluid. However, when a pump or other discharge means is being re-cocked,some suction may be applied to the nozzle assembly, typically to give anoperational pressure differential of about 0.2 to 0.5 bar across thenozzle assembly, although it is possible that a pressure differentialacross the nozzle assembly of up to 1 bar could be drawn during thesuction stroke of the pump. The nozzle assemblies of the inventionshould therefore be dimensioned so that the surface tension and otherflow restrictive effects prevent flow through the nozzle assembly when aminimum pressure differential of about 0.2 bar, preferably 1 bar, isapplied across the assembly. In order to provide a measure of safety,for example if the spray generating device is dropped or otherwisesubjected to sudden forces, it will usually be preferred that a pressuredifferential of up to 3 bar causes no significant flow of fluid throughthe nozzle assembly of the invention in the event that the device isdropped. The term ambient pressure differential is therefore used hereinto denote the pressure differential across the nozzle assembly, ie.between the exterior of the nozzle aperture and the upstream inlet tothe conduit, when the nozzle assembly or the spray generating deviceincorporating it is in its rest condition. Operational pressuredifferential is used herein to denote the pressure differential acrossthe nozzle assembly when the device is being re-cocked in preparationfor a subsequent discharge stroke of the spray generating device ofwhich the nozzle assembly forms part.

Although the ambient and operational pressure differentials are notsufficient to cause back flow of fluid through the nozzle assembly, whenthe spray generating device is operated, the fluid is pressurized, oftento up to 500 bars, to discharge the fluid as a spray of fine dropletsfrom the nozzle aperture. The high pressure differential across thenozzle assembly overcomes the surface tension and other flow restrictioneffects of the nozzle assembly and forces the fluid through the nozzleassembly. Typically, significant flow of fluid through the nozzleassembly to form a spray occurs in excess of about 50 bars pressuredifferential across the nozzle assembly of the invention, although aslow flow of fluid may occur at pressure differentials below this, forexample at above 10 to 25 bars.

The conduit(s) serving to restrict the back flow of fluid can beprovided by one or more fine bore tubes or conduits in the housing. Suchfine bores can be formed as bores leading radially from an annular feedgallery to the axial bore to the nozzle orifice 14 or can be axial boreswithin the housing, for example formed by laser drilling the bores in aplastic or similar nozzle block and securing a nozzle plate having theappropriate radial connecting grooves or bores to connect the fine boresto the nozzle aperture to the end face of the nozzle block.Alternatively, the flow restriction can be provided by constricting awider bore tube feeding fluid to the nozzle aperture.

However, it is preferred to form the conduit as a comparatively widebore chamber and to achieve the restriction of the back flow by locatingan infill member within the chamber. The infill member can be a flatplate with holes therethrough of the desired aperture size and shape, ora ceramic or other fritted or bonded material with a suitable foraminousor porous structure so that the infill member occupies the full width ofthe chamber and the fluid flows through the pores or apertures in theinfill member. However, it is preferred that the infill member be asolid or hollow plug which does not extend fully to the side or endwalls of the chamber so that the clearance gap between the infill memberand the side and/or end walls of the chamber form the requiredrestricted flow passageways. These passageway(s) can be radial, as whenthe infill member does not extend fully to the end of the chamber,and/or can be axial as when the clearance is between the side walls ofthe infill member and the chamber. However, it is within the scope ofthe present invention for the infill member to carry one or morecircumferential ribs or the like and for the clearance fit to be betweenthe radially outward extremities of these and the opposing wall of thechamber to provide the flow restriction(s) or vice versa where thechamber wall carries the circumferential ribs. Similarly the clearancebetween the transverse end wall of the chamber and the end face of theinfill member can be provided by the axially extreme faces of one ormore annular ridges carried by the chamber wall or the infill member.For convenience, the invention will be described hereinafter in terms ofopposing walls of the chamber and infill member which do not carry suchribs. Preferably, the passageway(s) are axial and for convenience theinvention will be described hereinafter in terms of an annular axialpassageway formed by the clearance gap between the side walls of thechamber and the infill member. It will be appreciated that thepassageway(s) can also be provided by axial grooves in the surface ofthe infill member. It is also preferred that the infill member beprovided with one or more radial ducts, for example grooves or ribs,which allow fluid to flow across the end faces of the infill member tothe annular passageway.

In a particularly preferred form of the nozzle assembly of theinvention, the conduit is provided as a blind ended axial chamber havingthe nozzle aperture located at or adjacent the blind end of the chamber,preferably in the transverse end wall of the chamber; and the infillmember is substantially congruent with the internal transverse end walland/or the axial side walls of at least the blind end of the chamber andis a clearance fit therein to form the passageway(s) between the opposedwalls of the chamber and the infill member.

It is particularly preferred that the chamber be cylindrical and thatthe infill member be a corresponding cylinder to form an annularpassageway between the internal radial wall of the chamber and theexternal radial wall of the infill member, although othercross-sectional shapes, for example triangular or hexagonal, may be usedif desired. For convenience, the invention will be described hereinafterin terms of a generally cylindrical housing having a circularcross-section chamber formed within it.

The optimum radial and axial dimensions for the flow restrictingpassageway(s) can readily be determined for any given case by simplecalculations from the rheological properties of the fluid and by simpletrial and error tests. Preferably, the minimum cross-sectional dimensionof the passageway(s) in the nozzle assembly, for example the clearancebetween the relevant walls of the infill member and the chamber, is lessthan the maximum dimension, for example the diameter, of the nozzleaperture, whereby the passageway(s) serves both as a flow restrictor toreduce back flow of the fluid and as a filter for the fluid flowingthrough the nozzle assembly. Typically, the passageway(s) will have aflow-transverse dimension of from 1 to 50 micrometers, notably less thanabout 20 micrometers, for example from 2 to 10 micrometers. The requireddimensions between the infill member and the walls of the chamber withinwhich it is located can be achieved by making the infill member a tightclearance fit within the chamber so that the roughness of the opposedsurfaces provides the necessary clearance fit.

We believe that flow restriction valves incorporating the chamber andinfill member concept described above are novel. The invention thereforealso provides a device for controlling the flow of a fluid, which devicecomprises:

a. a housing member having an internal chamber through which fluid isadapted to flow; and

b. a static infill member located within the chamber and forming apassageway for the flow of fluid between the internal wall of thechamber and the external wall of the infill member, which passageway isdimensioned so as to restrict the back flow of fluid therethrough at anambient pressure differential and at an operational pressuredifferential.

Preferably, the nozzle aperture is formed as an integral part of thehousing member within which the chamber and conduit are formed, forexample as an axial bore or conduit fed from the chamber within thehousing body. The nozzle aperture can take a number of forms, but ispreferably an aperture in a jewel or metal nozzle orifice member, forexample the transverse end wall of the chamber, through which the fluidis fed under pressure from the chamber. Preferably, the nozzle orificehas an aperture diameter of less than 10 micrometers, for example from 2to 6 micrometers. If desired, the nozzle orifice can be non-circular orthe nozzle assembly can incorporate a swirl chamber and/or other meansfor enhancing the production of fine droplets, for example droplets witha mass median diameter of less than 10 micrometers. Such other means canbe, for example, an impingement ball, plate, blade or other static orvibrating surface. Where a non-circular aperture is employed, it ispreferred that the ratio of the maximum radial dimension of the apertureto its minimum radial dimension be at least 2:1, eg. from 3:1 to 10:1,and that any angles in the lip of the aperture be sharp.

As indicated above, the nozzle assembly of the invention may act toseparate solid particles from the fluid passing through it where thepassageway(s) in the assembly are smaller than the maximum nozzleaperture dimensions. However, it may be preferred to incorporate one ormore separation means, for example a conventional fine aperture metalgauze filter mesh, notably one having a mesh aperture size in the range1 to 10 micrometers, to separate solid particles from the fluid upstreamof the passageway(s) in the nozzle assembly. Conveniently, suchseparation means are provided by a disc of suitable filter mesh which islocated within the chamber of the nozzle assembly immediately upstreamof the infill member and is supported by the upstream end face of theinfill member.

Thus, in a particularly preferred form of the nozzle assembly of theinvention, the assembly is formed from generally cylindrical housinghaving a blind ended cylindrical axial chamber substantially co-axiallytherein so that the nozzle assembly has radial symmetry; and the axialconfiguration is that the nozzle aperture is formed in the transverseend wall of the chamber, the infill member is located within the chamberand immediately adjacent the transverse end wall of the chamber, theseparation means is located transversely and adjacent the upstream faceof the infill member and the open end of the housing is crimped over orprovided with other means whereby the assembly is retained as a unitaryconstruction.

As indicated above, the nozzle assembly of the invention finds especialuse with spray generating devices. The exact nature, form ofconstruction and method of operation of the spray generating device canbe of any suitable type, for example a pressurized or liquefied gaspropellant aerosol can type device. However, the invention is ofespecial use with mechanically actuated devices in which a measured doseof fluid is subjected to an increase in pressure to expel the fluidthrough the nozzle assembly of the invention.

The MDI (metered dose inhaler) shown in FIG. 6 comprises a body in whichthere is defined a cylinder 2 of circular cross-section, in which apiston 3 is mounted for reciprocating movement. The cylinder 2communicates with a pressure chamber 4 of reduced cross-section. Thepiston 3 has a reduced diameter portion 5 which sealingly engages withinthe pressure chamber 4, by means of a plastic (e.g., PTFE or Nylon)sealing cap or ring provided on the piston portion 5. The seal may beformed integrally with the reduced diameter portion 5 of the piston--forexample, as a cap, rib or bead.

A pre-loaded compression spring 6 is located in the cylinder 2, betweenthe enlarged head of the piston 3 and an opposite end wall of thecylinder 2. An operating rod 631 is connected to the piston 3, andpasses through the spring 6 and through a passageway 34 in the body 1,to protrude from the body 1. At or adjacent an end of the rod 631 thereis provided a handle means 32 for moving the rod 631 and the piston 3.If desired, the end of rod 631 can be connected to a trigger mechanismor lever mechanism incorporating a mechanical advantage so that the usercan readily operate the device against the compressive force of spring6. A latching means 33 provided on the body 1 engages with the rod 631,to latch the rod 631 in a loaded position, as illustrated in FIG. 6. Anactuating button 35 is provided, for releasing the latching means 33.

Also defined within the body 1 is a cavity 65 in which there is locateda collapsible bag 60 containing the product to be dispensed (e.g., aliquid drug). A door 66 on the side of the body 1 may be opened, inorder to exchange the collapsible bag 60. By means of a connector 62,the interior of the bag 60 communicates with an inlet passage 61 which,in turn, communicates with the pressure chamber 4 via a non-return valve63.

Also connected to the pressure chamber 4 is an outlet passage 621 whichextends from the pressure chamber 4 to an atomizing head 22, via anon-return valve 23 and a pressure release valve 25.

Optionally, the body 1 is provided with a mouthpiece 40, which affordsan atomization chamber around the atomizing head 22.

In use of the MDI of FIG. 6, when the piston 3 is in the loaded positionas illustrated in FIG. 6, the pressure chamber 4 is full of liquid whichhas been supplied from the bag 60, via the passage 61 and non-returnvalve 63. The compression spring 6, as mentioned above, is alreadypre-loaded when fitted in the cylinder 2. The loading of the spring isincreased further by withdrawing the rod 631 and thereby the piston 3 tothe loaded position that is illustrated in FIG. 6.

The rod 631 is latched in its loaded position as illustrated in FIG. 6,by the latching means 33. Upon depressing the actuating button 35, thelatching means 33 is released, thereby allowing the piston 3 to movesuddenly forward under the force of the compression spring 6, to imparta sudden pressure pulse to the liquid in the pressure chamber 4.

The pressure in the liquid in the pressure chamber 4 therefore quicklybuilds up to exceed the limit value of the pressure release valve 25,and the liquid is then ejected under high pressure through the outletpassage 621 to the atomizing head 22, via the one-way valve 23. Duringthe forward travel of the piston 3, the non-return valve 63 preventsliquid from being returned to the bag 60, via the inlet passage 61. Asthe liquid is ejected through the atomizing head 22, it is atomized intoa fine spray, which can then be inhaled. The optional mouthpiece 40provides an atomization chamber within which the fine spray is enclosed,and facilitates the inhalation of the spray.

To reload the MDI, the rod 631 is pulled back by means of the handle 32against the resilient bias of the spring 6 and, at the end of itstravel, the latching means 33 automatically latches the rod 631 into alatched end position. During this travel of the piston 3, liquid issucked out of the collapsible bag 60 into the pressure chamber 4, viathe inlet passage 61 and one-way valve 63. At this time, the one-wayvalve 23 prevents air being sucked into the pressure chamber 4 via theoutlet passage 621. Due to the latching of rod 631, the fluid inpressure chamber 4 is held at ambient pressure and there is little or norisk of loss of fluid from the chamber. The operation of latching means33 provides the user with a clear indication when piston 3 has completedthe desired travel within cylinder 2 and that the required dose of fluidhas been taken up. If the user fails to withdraw rod 631 to a sufficientextent, the latching means 33 will not engage and the user will detectthe spring bias from spring 6 and will know to withdraw rod 631 further.The latching means 33 thus provides both the means for holding fluid inchamber 4 under ambient pressure and a means for alerting the user toincomplete operation of the device, hence reducing the risk of variableoperation of the device.

Thus, the MDI is again in a loaded position, as illustrated in FIG. 6,ready for firing.

DESCRIPTION OF THE DRAWINGS

To aid understanding thereof, the invention will now be described withrespect to a preferred form thereof as shown in the accompanyingdrawings, in which FIG. 1 is a diagrammatic axial cross-section throughone form of the nozzle assembly of the invention; FIGS. 2, 3 and 4 areaxial cross-sections through alternative forms of the nozzle assembly;and FIG. 5 is an axial plan view of an alternative form of the infilldevice for use in the assembly of FIG. 1. FIG. 6 shows a spraygenerating device configured with the nozzle assembly of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The device of the invention, notably that shown in FIG. 1, is ofparticular use in the atomization of aqueous solutions of medicaments,notably in measured dose inhalation devices (MDI's). For convenience theinvention will be described in respect of a device for such use.

The device comprises a main hollow generally cylindrical housing body102 having one end closed by a transverse end wall 104 to define a blindended chamber located substantially co-axially within it. The closed endwall 104 is provided with a fine bore nozzle aperture 106 directedgenerally axially and located with its axis substantially co-incidentwith the longitudinal axis of the body 102. A transverse filter mesh 110is located within the open end of body 102 and is held within the bodyby folding over the exposed lip of the body 102 to form an annularretaining flange 112 as shown. This also forms the axial entry port 126to the chamber within body 102. A plastic sealing ring or gasket 114 orthe like is located between said flange 112 and the filter 110.

A cylindrical infill member 116 is located substantially co-axiallywithin the chamber within the body between the filter 110 and the endwall 104. This cylinder is formed with its radially outward facesubstantially congruent to the interior wall of the chamber. Theupstream end face of the cylinder 116 acts to support the filter 110.One or more radial grooves or ribs 120 and 122 are formed in both endfaces of the cylinder 116 to allow the passage of fluid from the entryport 126 to the nozzle aperture 106. An annular passageway is formedbetween the radially outward wall of cylinder 116 and interior wall ofthe chamber in body 102 to allow fluid to flow past the cylinder 116.The flange 112 is folded into place after assembly of the cylinder 116,filter 110 and gasket 114, to retain the nozzle assembly as a unitarywhole in which the cylinder 116 is retained against axial movementwithin the chamber of body 102.

The body 102 is securely held in position on the MDI or other spraygenerating device by any suitable means, for example by means of acrimped over sleeve extension 130 to the body of the spray generatingdevice. Alternatively, the body 102 can be screw threaded, bayonetfitted, welded or otherwise secured to the body of the spray generatingdevice, for example to the valve outlet stem of a pressurized container.

The clearances between the end faces of the cylinder and the filter 110and the transverse end wall 104 and/or the clearance between theradially outward wall of the cylinder and the inner wall of the chamberare selected so that the ambient and operational pressure differentialexperienced between nozzle aperture and the inlet 126 will not besufficient to cause a back flow of fluid from the nozzle aperture to theinlet 126. Typically, the clearance is also selected so that it will actto filter out particles which pass through filter mesh 110 so that thenozzle aperture 106 is not blocked by them. Thus, for a 5 micrometernozzle aperture, it will usually be preferred that the radialpassageways 120 and 122 have an axial dimension of from 1 to 4micrometers, notably about 2.5 micrometers. Such dimensions for theradial passages also provide an adequate restriction on back flow undermost conditions. Where the annular passageway 128 is to provide the backflow restriction, similar radial dimensions for the annular clearancehave been found to give satisfactory results both as a filter and torestrict back flow. Such clearances can conveniently be achieved by arough finish to the interior walls of the chamber within the body 102and/or to the exterior of cylinder 116. Thus, if the cylinder is a pushfit within the housing and can just be rotated manually therein, theclearance is typically as required by the present invention.

In operation of the spray generating device, a metered dose of themedicament or other fluid is applied under pressure to inlet 126,typically at from 100 to 400 bars. This overcomes the surface tensionand drag effects in the nozzle assembly and forces fluid to flow via theradial grooves 120 into the annular axial passageway 128 and then viaradial grooves 122 to the nozzle aperture 106. When the spray has beendischarged, there is no significant pressure differential between thechamber within the assembly and the ambient environment downstream ofthe nozzle aperture. If anything, there is a slight positive pressurewithin the chamber due to the restriction to free flow achieved by thenozzle assembly. Back flow of fluid to inlet 126 from the nozzleaperture 106 is substantially prevented due to the small dimensions ofthe grooves 120, 122 and the annular passageway 128.

When the spray generating device is re-loaded for a subsequentoperation, a negative pressure of no more than approximately 1 bar maxvacuum is generated at the entry 126 as the measured dose of fluid isdrawn into the measuring chamber (not shown) by retraction of a pistonin a cylinder or other means. However, the flow restriction imposed bythe combined passageway formed by the grooves 120 and 122 and theannular passageway 128 prevents the pressure differential between thenozzle aperture and the inlet 126 from moving any fluid in saidpassageway remaining from the previous discharge operation of the spraygenerating device. However, the large positive pressure generated whendispensing the fluid is sufficient to overcome the surface tensionforces and other flow restrictions to ensure that the fluid is dispensedas a spray from the nozzle aperture.

In the variation of the nozzle assembly 10, shown in FIG. 2, the filtermesh is omitted and the annular passageway 13 between the cylinder 12and the chamber wall 11 provides an effective filter for solid particleswhere the radial dimension of the passageway 13 is about half thediameter of the nozzle aperture 14 formed in the end face 16. Again theradial passage(s) 15 between the end wall 16 and the end face of thecylinder 12 may be fine to assist the operation of the annularpassageway or may be large enough to have little or no back flowrestriction effect. The clearance between the cylinder 12 and the wall11 works both as a filter and as a non-return valve.

In the variations shown in FIGS. 3 and 4, the clearance is providedbetween as the radial clearance 21 between a radial projection, forexample a circumferential rib 20, on the cylinder 12 and the axial wall11 of the chamber (in FIG. 3); or as the axial clearance 31 between anannular axially extending rib 30 carried by the end face of the cylinder12 (in FIG. 4). The ribs shown in FIGS. 3 and 4 could be carried by thechamber walls 11 and/or 16 and not upon the cylinder 12 as shown.

In the form of nozzle assembly as shown in FIG. 5, the cylinder 12 isformed as a composite structure from a series of annular sleeves 41, 42mounted co-axially upon one another with the inner sleeve mounted upon asolid cylinder 48. Annular clearances 43 and 49 between each sleeve andthe next provide a number of axial passageways in the overall cylinderconstruction which act in the same way as the annular passageways 13 or21 in FIGS. 3 and 4.

With water based solutions, the fluid is applied to the inlet 126 of thenozzle assembly of FIG. 1 at a pressure of between 100 and 400 bars. Fora nozzle aperture of mean diameter of 5 micrometers, the nozzle assemblywill filter out particles above about 2.5 micrometers size with anannular gap 128 of about 2.5 micrometers. Where the annular gap 128 inthe nozzle assembly is not to act as a filter, but the nozzle assemblyrelies upon the filter 110 to remove solid particles, the annular gap128 can be larger, for example as much as 50 micrometers. With thesepressures and dimensions, we have found it sufficient to use roughsurfaces at the faces of the cylinder to act as the fluid grooves 120and 122. Likewise the annular passageway 128 can be formed by theroughness of the surface finish of the body 102 and cylinder 116.

We claim:
 1. A spray generating device, comprising:pressurizing meansfor applying a predetermined amount of energy to a metered quantity offluid in order to subject the fluid to a predetermined increase inpressure by transitioning from a loaded configuration to a restconfiguration; retaining means for retaining said pressurizing means insaid loaded configuration; releasing means for releasing said retainingmeans so that said pressurizing means transitions from said loadedconfiguration to said rest configuration to thereby subject the fluid tosaid predetermined increase in pressure; and a nozzle assembly whichcomprises a nozzle aperture for discharge of the fluid as a spray ofdroplets, and a conduit in fluid flow communication with said nozzleaperture, wherein the flow of fluid through said conduit is restrictedby the minimum effective cross-sectional area of said conduit transverseto the line of flow of fluid through said conduit, wherein transitioningsaid pressurizing means from said rest configuration to said loadedconfiguration creates suction, said suction being insufficient to causeback-flow of fluid from said nozzle aperture into said conduit.
 2. Aspray generating device as claimed in claim 1, wherein said conduitcomprises a fine bore tube and said effective cross-sectional area isthe transverse cross-section of said fine bore tube.
 3. A spraygenerating device as claimed in claim 2, further comprising: a filterelement.
 4. A spray generating device as claimed in claim 1, whereinsaid conduit comprises a chamber of greater cross-sectional area thansaid nozzle aperture, and further comprising a static infill memberlocated within said conduit so as to occupy at least part of thecross-section of said chamber.
 5. A spray generating device as claimedin claim 4, wherein a first passageway is formed between an end wall ofsaid conduit and an end wall of said infill member.
 6. A spraygenerating device as claimed in claim 5, wherein said chamber comprisesan axial chamber having a blind end, said nozzle aperture locatedadjacent said blind end of said axial chamber, and said infill member issubstantially congruent with an internal transverse end wall of saidblind end of said axial chamber and is a clearance fit therein to formsaid first passageway.
 7. A spray generating device as claimed in claim5, wherein a second passageway is formed between a side wall of saidconduit and a side wall of said infill member.
 8. A spray generatingdevice as claimed in claim 7, wherein said chamber comprises an axialchamber having a blind end, said nozzle aperture located adjacent saidblind end of said axial chamber, and said infill member is substantiallycongruent with an internal side wall of said blind end of said axialchamber and is a clearance fit therein to form said second passageway.9. A spray generating device as claimed in claim 7, wherein flow offluid through said nozzle assembly is substantially prevented when apressure differential substantially equal to 0.2 bar is applied acrosssaid nozzle assembly.
 10. A spray generating device as claimed in claim7, wherein the minimum cross-sectional dimension of said secondpassageway is less than the maximum dimension of said nozzle aperture,wherein said second passageway serves both as a flow restrictor toreduce back flow of fluid and as a filter for said fluid flowing throughsaid nozzle assembly.
 11. A spray generating device as claimed in claim5, wherein the minimum cross-sectional dimension of said firstpassageway is less than the maximum dimension of said nozzle aperture,wherein said first passageway serves both as a flow restrictor to reduceback flow of fluid and as a filter for fluid flowing through said nozzleassembly.
 12. A spray generating device as claimed in claim 1, whereinflow of fluid through said nozzle assembly is substantially preventedwhen a pressure differential substantially equal to 0.2 bar is appliedacross said nozzle assembly.
 13. A spray generating device as claimed inclaim 1, further comprising: a filter element.
 14. A method ofdischarging a fluid as a spray of droplets, comprising the stepsof:loading a pressurizing means by transitioning said pressurizing meansfrom a rest configuration to a loaded configuration; retaining saidpressurizing means in said loaded configuration; transitioning saidpressurizing means from said loaded configuration to said restconfiguration to thereby subject a metered quantity of fluid to apredetermined increase in pressure; and flowing the fluid through anozzle assembly which comprises a nozzle aperture for discharge of thefluid as a spray of droplets, and a conduit in fluid flow communicationwith said nozzle aperture, wherein the flow of fluid through saidconduit is restricted by the minimum effective cross-sectional area ofsaid conduit transverse to the line of flow of fluid through saidconduit, wherein said loading step creates suction, said suction beinginsufficient to cause back-flow of fluid from said nozzle aperture intosaid conduit.
 15. A method as claimed in claim 14, wherein said flowingstep is performed so that the flow of fluid through said nozzle assemblyis substantially prevented when a pressure differential substantiallyequal to 0.2 bar is applied across said nozzle assembly.
 16. A spraygenerating device, comprising:a spring-loaded pump mechanism, whereintransitioning said pump mechanism from a loaded configuration to a restconfiguration applies a predetermined amount of energy to a meteredquantity of fluid in order to subject the fluid to a predeterminedincrease in pressure and transitioning said pump mechanism from saidrest configuration to said loaded configuration creates suction;retaining means for retaining said pump mechanism in said loadedconfiguration; releasing means for releasing said retaining means sothat said pump mechanism transitions from said loaded configuration tosaid rest configuration to thereby subject the fluid to saidpredetermined increase in pressure; and a nozzle assembly whichcomprises a nozzle aperture for discharge of the fluid as a spray ofdroplets, and a conduit in fluid flow communication with said nozzleaperture, wherein the flow of fluid through said conduit is restrictedby the minimum effective cross-sectional area of said conduit transverseto the line of flow of fluid through said conduit, wherein the suctioncreated by transitioning said pump mechanism from said restconfiguration to said loaded configuration is insufficient to causeback-flow of fluid from said nozzle aperture into said conduit.